1. Technical Field
The present invention relates generally to cellular wireless networks; and more particularly to the characterization of radio frequency propagation in a cellular wireless network and use of such characterization in radio frequency channel reuse planning.
2. Related Art
Cellular wireless networks are generally known to include a “network infrastructure” that facilitates wireless communications with mobile stations operating within a respective service coverage area. The network infrastructure typically includes a plurality of base stations dispersed throughout the service coverage area, each of which supports wireless communications within a respective cell (or set of sectors). The base stations couple to base station controllers (BSCs), with each BSC serving a plurality of base stations. Each BSC couples to a mobile switching center (MSC), which also couples to the PSTN, the Internet and/or to other MSCs.
A wireless mobile station operating within the service coverage area communicates with one or more of the base stations. The base stations route the communications to the serving MSC via a serving BSC. The MSC routes the communications to another subscribing wireless unit via a BSC/base station path (which may be the same BSC/base station path when the communications are with another subscribing unit serviced by the same base station) or via the PSTN/Internet/other network to terminating destination.
Various interface standards have been developed to standardize wireless communications so that equipment of differing vendors may interface. Wireless communication interface standards include, for example, the Advanced Mobile Phone Service (AMPS) standards, the Global Standards for Mobility (GSM), the Code Division Multiple Access (CDMA) and the Time Division Multiple Access (TDMA) standards. These operating standards set forth the technical requirements that facilitate compatible operation between equipment of differing vendors.
The government allocates the system operator a radio frequency (RF) spectrum within which to support all wireless communications in the service coverage area. This RF spectrum is then subdivided into a plurality of RF channels, each RF channel centered about a respective carrier frequency and separated from adjacent carrier frequencies by a separation bandwidth. Each RF channel is able to carry a maximum traffic limit, the limit dependent upon the characteristics of the RF channel.
In order to increase the capacity of the system, RF channels are reused across the cellular wireless communication system. Each cell is allocated one or more particular RF channels, the number of which is a function of load. The number of available RF channels is limited, however, and in order to satisfy system-loading requirements, RF channels are reused across the system. Such reuse of RF channels, results in interference between cells using the same RF channel.
Because of interference between cells/sectors using the same RF channel, sufficient RF isolation must exist between the cells/sectors. RF isolation is typically referred to as a carrier to interference (C/I) ratio and expressed in decibels. The RF isolation between cells/sectors is dependent upon a number of factors including topology, geography, base station placement, presence or absence of obstacles, and other factors. A larger C/I ratio indicates better RF isolation between a cell/sector. The ability to reuse RF channels depends upon the RF isolation between cells/sectors. An “Isolation Matrix” includes the RF isolation of each cell/sector pair in the cellular wireless network or a subset of cells/sectors in the cellular wireless network. This Isolation Matrix may be employed to determine a frequency reuse pattern that is deployed within the cellular wireless network.
Prior techniques for determining components of the Isolation Matrix included what is referred to as “drive testing.” With drive testing measurement, an RF channel corresponding to a cell/sector under test is “keyed up” to a particular transmission level. A drive-testing vehicle equipped with an RF receiver and a positioning system, such as a Global Positioning System (GPS) device, is then used to measure the strength of the keyed up RF channel and the associated interference in a plurality of other cells/sectors across the system. This procedure is then repeated for other cells/sectors in the system until enough data has been collected to create the Isolation Matrix. This technique is expensive, slow and ties up RF channels all during the testing.
However, such drive testing suffered numerous shortcomings. To obtain a good set of data, several attempts to collect the data were generally required. Further, drive testing required 100% coverage of the network, which took considerable effort and expense to achiever. Because drive testing procedures required that a particular RF channel be made unavailable across the system, due to reduced system capacity during the drive testing, substantial revenue was lost during the testing. Also, since all measurements were made from a vehicle, only in-car coverage was included and no in-building or pedestrian traffic was considered. Moreover, since drive testing was performed over several weeks (or even months) network characteristics changed during the testing period. Finally, the drive testing effort required extensive planning and scheduling of resources.
Thus, there is a need in the art for a system and method that may be employed to gather data for the creation of an Isolation Matrix for use in RF reuse planning. This system and method should be simple, efficient, and inexpensive and consume a minimum of system resources.